PLANT RESISTANCE Virulence Analysis of Hessian Fly Populations From Texas, Oklahoma, and Kansas

In recent years, the number of wheat, Triticum aestivum L., Þelds heavily infested by Hessian ßy, Mayetiola destructor (Say), has increased in the Great Plains of the United States. Historically, resistance genes in wheat have been the most efÞcient means of controlling this insect pest. To determine which resistance genes are still effective in this area, virulence of six Hessian ßy populations from Texas, Oklahoma, and Kansas was determined, using the resistance genes H3, H4, H5, H6, H7H8, H9, H10, H11, H12, H13, H16, H17, H18, H21, H22, H23, H24, H25, H26, H31, and Hdic. Five of the tested genes, H13, H21, H25, H26, and Hdic, conferred high levels of resistance ( 80% of plants scored resistant) to all tested populations. Resistance levels for other genes varied depending on which Hessian ßy population they were tested against. Biotype composition analysis of insects collecteddirectly fromwheatÞelds inGraysonCounty,TX, revealed that theproportionof individuals within this population virulent to the major resistance genes was highly variable (89% forH6, 58% for H9, 28% forH5, 22% forH26, 15% forH3, 9% forH18, 4% forH21, and 0% forH13). Results also revealed that the percentages of biotypes virulent to speciÞc resistance genes in a given population are highly correlated (r 0.97) with the percentages of susceptible plants in a virulence test. This suggests that virulence assays, which require less time and effort, can be used to approximate biotype composition.